The present invention relates to an electric fuse to be attached to a printed circuit board when it is used.
To control various electric devices, an electronic circuit is often used, which is formed of various electronic components mounted on a printed circuit board and connected to each other. In such an electronic circuit provided on a printed circuit board, it is possible that a large amount of current will flow through a circuit component due to, for example, a failure thereof, whereby the component will generate a lot of heat or be broken, thereby causing the electric device to malfunction.
To avoid this, in such an electronic circuit, an electric fuse is provided on the printed board circuit. When a lot of circuit current flows, the electric fuse blows to interrupt the circuit current, thereby preventing generation of a lot of heat and abnormal operation of an electric device as described above. However, if the pressure in a fuse case exceeds a predetermined value when the fuse blows, the case will almost burst out. This may damage another portion in the electric device.
An electric fuse used for the aforementioned purpose is disclosed in, for example, U.S. Pat. No. 5,179,436 invented by Asdollahi et al. Usually, the fuse case includes a base and a cap, and the cap is blown away from the base when the pressure in the case exceeds a predetermined value. To prevent this, the electric fuse invented by Asdollahi et al. has a case airtightly sealed with its internal pressure kept at a certain value, and a predetermined braking point is set. When a dangerous peak pressure is created, an opening is formed in the case at the predetermined braking point, through which a gas of a sufficient volume is exhausted to reduce the pressure in the case.
Since, however, an electric fuse of this type has a small outer size and a small inner volume, if it has a case airtightly sealed with its internal pressure kept at a certain value, the amount of oxygen contained in the case for burning its fuse element is inevitably small. This results in variations in the time required for blowing the fuse element between different fuse products.
Furthermore, since the base and the cap are formed of plastic and engaged with each other by fitting projections in locking grooves, it is possible that the case will fly asunder due to a high-temperature and high-pressure gas which will occur when, for example, the fuse element blows. Moreover, it is possible that an insulation failure will occur due to carbonation of the inner surfaces of the cap and the base.
It is an object of the present invention to provide an electric fuse, which does not show a wide range of variations in the time required for its fuse element to blow.
It is another object of the invention to provide an electric fuse, in which its case is prevented from flying asunder even if a high-temperature and high-pressure gas occurs when, for example, its fuse element blows, and in which an insulation failure due to carbonation of the inner surfaces of the cap and the base of the case is prevented.
To attain the objects, the invention provides an electric fuse comprising: a base; a pair of electrodes provided through the base; a fuse element connected to the electrodes; a cap engaged with the base and constituting a case together with the base, the case containing the fuse element; and at least one air pass hole formed in the case for permitting air to always flow between an inside and an outside of the case.
Since the electric fuse has at least one air pass hole for permitting air to always flow between the inside and the outside of the case, the case is not sealed but outside air always flows into it. Accordingly, sufficient oxygen is supplied into the case, and hence the time required for blowing the fuse element can be shortened and controlled to fall within a small range of variations.
Furthermore, since a high-temperature and high-pressure gas that is generated when blowing the fuse element is discharged to the outside of the case through the at least one air pass holes, the case is prevented from flying asunder, and an insulation failure is prevented which may occur due to carbonation of the inner surfaces of the base and the cap.
Preferably, the at least one air pass hole is formed in the base.
More preferably, the base is in the form of a rectangle having long sides and short sides when viewed from above, and the at least one air pass hole is formed along one of the long sides of the base.
Further preferably, the base is in the form of a rectangle having long sides and short sides when viewed from above, and the at least one air pass hole is formed along one of the short sides of the base.
Yet preferably, the at least one air pass hole is formed in a substantially central portion of the base.
Also preferably, the at least one air pass hole is formed in a side surface of the cap.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.